Virus is the cause of various infectious disorders. For example, members of the coronavirus family cause hepatitis in mice, gastroenteritis in pigs, and respiratory infections in birds and humans. Among the more than 30 coronaviruses isolated so far, three or four infect humans. The severe acute respiratory syndrome (SARS), a newly found infectious disease, is associated with a novel coronavirus. This life-threatening respiratory coronavirus touched off worldwide outbreaks in 2003. Vaccines and drugs against SARS coronavirus are being vigorously sought. Nevertheless, the progress is rather slow due to safety concerns.